1. Field of the Invention
The invention relates to an electric power steering system that includes a motor that applies assist force to a steering system.
2. Description of Related Art
In an electric power steering system described in Japanese Patent Application Publication No. 2004-66999 (JP-A-2004-66999), a map that defines the correlation between a motor current and a motor resistance is prepared in advance, in view of the fact that an actual resistance of a motor changes depending on a current applied to the motor. A motor resistance is calculated by applying a motor current at that moment to the map. The calculated motor resistance is set as a new motor resistance, and then the set motor resistance is used to execute various computations.
The motor resistance changes under the influence of motor current and outside air temperature. Therefore, it is necessary to take into account induced voltage in order to accurately calculate the motor resistance. That is, induced voltage influences the estimation accuracy of the motor resistance, and the estimation accuracy decreases with an increase in induced voltage. Therefore, it is desirable to calculate the motor resistance with this point taken into account.
However, in existing systems including the electric power steering system described in JP-A-2004-66999, the above point is not particularly taken into account. Therefore, there is a possibility that a newly set motor resistance significantly deviates from an actual motor resistance as the motor resistance is updated.